Fluidfoil fence

ABSTRACT

A fluidfoil  32, 70  comprising a main body  38, 84  and a fence  40, 72,  wherein the fence  40, 72  has different lean angles with respect to the main body  38, 84  at different chordwise positions.

The present disclosure concerns fluidfoils, gas turbine engines andmethods of managing flow tip momentum transfer. More specifically thedisclosure concerns, in part, particular fluidfoil fence arrangements.The disclosure may have particular utility when applied to some or allof the propeller blades of open rotor gas turbine engines havingcontra-rotating propeller stages, but this is not intended to belimiting.

It is known to provide the tips of fluidfoils with fences in the form ofan aerodynamic fence at the end of the fluidfoil. The fence is leant ata particular lean angle, either towards the suction side of thefluidfoil (dihedral fence) or towards the pressure side of the fluidfoil(anhedral fence). Some fluidfoil tips may also have both an anhedral anda dihedral fence.

Fences increase efficiency by reducing fluid momentum in the radialdirection as it passes the fluidfoil.

Fences also serve to reduce vortices at the fluidfoil tips. Vortices areinefficient and can be acoustically undesirable, especially wheredownstream structures intercept the vortices. A fence inhibits the flowof fluid from the pressure side of the fluidfoil (where the pressure isgreater) to the suction side (where the pressure is lower). This tendsto limit or prevent the creation of a spiralling trail of fluid (vortex)leaving the fluidfoil tip.

Where however momentum transfer from the pressure side to the suctionside is insufficient in view of the presence of a fence, a vortexbreakdown occurs and a bubble is formed. Vortex breakdown is an abruptchange of flow structure that occurs in swirling flows. If the fencecauses sufficient reduction in momentum transfer, there may be a rapiddeceleration of the flow in the axial direction in the vicinity of thefence. Further there may be a rapid expansion of the vortex core atapproximately 50% of the tip chord, giving rise to a bubble likestructure. Although the breakdown causes the vortex to be weaker, itdoes not imply the decay of vorticity itself. The swirling is in factredistributed over a larger area, potentially worsening the downstreamwake. Vortex breakdown tends therefore to be even less desirable than astandard vortex from an acoustic viewpoint, particularly tending toincrease broadband noise. Moreover, the overall direction of axial flowis usually reversed inside the bubble, thereby compromising, to someextent, aerodynamic performance.

Unless otherwise specified, the word axial is used throughout thisspecification to refer to the direction of the main rotational axis of agas turbine engine to which fluidfoils would, in use, be attached.Similarly the word radial is used with respect to a gas turbine engineto which fluidfoils would, in use, be attached. Specifically radialrefers to directions perpendicular to the main rotational axis of thegas turbine engine.

According to a first aspect of the invention there is provided afluidfoil comprising a main body and a fence, wherein the fence hasdifferent lean angles with respect to the main body at differentchordwise positions.

The lean angle corresponds to the approximate angle that the fence orfence portion makes with a main body of the fluidfoil. More specificallythe lean angle at a particular chordwise position may be defined as: 90°plus the angle subtended by:

-   -   i) a normal to the surface at a point on the surface towards        which the fence portion leans, the point being at the tip of a        main body of the fluidfoil and at the particular chordwise        position, and    -   ii) a straight line between a tip of the fence at the particular        chordwise position and the point defined in i).

A fence with lean angle variation from the front to the back of thefence (in the axial direction) may offer the potential for control ofthe flow tip momentum transfer between the pressure and suction sides ofthe fluidfoil. By deliberate design of the lean angle variation it maybe possible to control the evolution of a leakage vortex, retaining itat the incipience of a breakdown event (i.e. low energy vortex). Such afence may therefore give the efficiency benefits of a standard fence,without instigating a potentially detrimental vortex breakdown.

In some embodiments the fence is divided along its chordwise extent intoat least two portions, where a first portion nearer a leading edge ofthe fence has a smaller lean angle than a second portion further fromthe leading edge. The first portion may reduce momentum transfer toreduce the tip vortex energy towards a critical condition of incipientvortex breakdown. Thereafter the second portion may prevent breakdownthrough a controlled increase in momentum transfer brought about by itsincreased lean angle.

In some embodiments the first portion is the nearest portion to theleading edge.

In some embodiments the first and second portions are immediatelyadjacent.

In some embodiments a third portion, further from the leading edge thanthe second portion, has a greater lean angle than the second portion.The third portion may therefore stabilise momentum transfer close to thebreakdown point and prevent an increase in vortex energy from a trailingedge of the second portion.

In some embodiments the second and third portions are immediatelyadjacent.

In some embodiments a first portion, nearest a leading edge of thefence, has the same lean angle as a second portion immediately adjacentthe first portion and the second portion is shorter than the firstportion. Shortening the second portion may provide a convenientalternative to increasing its lean angle with respect to the firstportion. Shortening a portion may have similar results to increasing itslean angle, as the impediment to flow from the pressure to the suctionside of the fluidfoil will be correspondingly reduced.

In some embodiments a third portion, immediately adjacent the secondportion, has a greater lean angle than the first and second portions.The third portion may therefore stabilise momentum transfer close to thebreakdown point and prevent an increase in vortex energy from a trailingedge of the second portion.

In some embodiments at least two portions are located at differentspanwise positions along the fluidfoil.

In some embodiments the lean angle within at least one portion issubstantially consistent. This may facilitate easier manufacturing ofthe fluidfoil. It may also make the momentum transfer difference thatwill occur as a consequence of that portion more predictable.

In some embodiments the lean angle within at least one portion varies.This may allow closer control over vortex energy.

In some embodiments there is a step-wise discontinuity between at leasttwo of the portions. This may be easier to manufacture than providingthe portions with complimentary ramp regions to join them.

In some embodiments at least two portions have complimentary rampregions which meet to smoothly join the portions. This may reduce stressoccurring between portions.

In some embodiments there is continuous smooth variation in the leanangle across the fence in the chordwise direction. In this case it mightbe considered that there are an infinite number of portions each with adifferent lean angle.

In some embodiments the fence has different spanwise lengths atdifferent chordwise positions. It may be for example that where thefence is divided into portions as previously described, each portion hasa consistent spanwise length, but at least two portions have differentspanwise lengths. Alternatively it may be that there is continuoussmooth variation in the spanwise length across the fence in thechordwise direction. Fence spanwise length variation may be of furtherassistance in managing the degree of impediment to momentum transfer andtherefore vortex energy.

In some embodiments the fluidfoil is an open-rotor propeller blade. Inother embodiments however the fluidfoil may for example be a blade orrotor blade, or specifically a ducted fan blade, a compressor blade, aturbine blade or a propeller blade.

According to a second aspect of the invention there is provided a gasturbine engine comprising one or more fluidfoils in accordance with thefirst aspect.

In some embodiments the gas turbine engine comprises contra-rotatingforward and aft propeller stages, where the forward propeller isprovided with at least one of the fluidfoils. The fluidfoil may beparticularly advantageous when used in the forward propeller stage of acontra-rotating propeller system. The potential control exerted over thevortex produced by the forward propeller fluidfoils may mean that theimpulsiveness with which the aft propeller intersects the vortex isdecreased, with consequent acoustic benefits.

In some embodiments the gas turbine engine is an open rotor engine.

According to a third aspect of the invention there is provided a methodof managing the flow tip momentum transfer from the pressure side to thesuction side along the tip of a fluidfoil comprising providing differentdegrees of impediment to fluid flow from the pressure side to thesuction side at different points along the tip in a chordwise direction.

In some embodiments the method further comprises arranging theimpediments so that in use they cause an approach to incipient vortexbreakdown and maintain this condition.

In some embodiments the method further comprises providing a firstupstream impediment to fluid flow from the pressure side to the suctionside and providing a second reduced impediment downstream of this.Optionally the method further comprises providing a third increasedimpediment downstream of the second reduced impediment.

The skilled person will appreciate that a feature described in relationto any one of the above aspects of the invention may be applied mutatismutandis to any other aspect of the invention.

Embodiments of the invention will now be described by way of exampleonly, with reference to the Figures, in which:

FIG. 1 is a sectional side view of a gas turbine engine havingcontra-rotating propeller stages;

FIG. 2 is a perspective view of a fluidfoil according to an embodimentof the invention;

FIG. 3 is a perspective view of a fluidfoil according to an embodimentof the invention.

Referring to FIG. 1, a twin-spooled, contra-rotating propeller gasturbine engine is generally indicated at 10 and has a principalrotational axis 9. The engine 10 comprises a core engine 11 having, inaxial flow series, an air intake 12, an intermediate pressure compressor14, a high-pressure compressor 15, combustion equipment 16, ahigh-pressure turbine 17, an intermediate pressure turbine 18, a freepower (or low-pressure) turbine 19 and a core exhaust nozzle 20. Anacelle 21 generally surrounds the core engine 11 and defines the intake12, nozzle 20 and a core exhaust duct 22. The engine 10 also comprisestwo contra-rotating propeller stages 23, 24 attached to and driven bythe free power turbine 19 via shaft 26.

The gas turbine engine 10 works in a conventional manner so that airentering the intake 12 is accelerated and compressed by the intermediatepressure compressor 14 and directed into the high-pressure compressor 15where further compression takes place. The compressed air exhausted fromthe high-pressure compressor 15 is directed into the combustionequipment 16 where it is mixed with fuel and the mixture combusted. Theresultant hot combustion products then expand through, and thereby drivethe high-pressure, intermediate pressure and free power turbines 17, 18,19 before being exhausted through the nozzle 20 to provide somepropulsive thrust. The high-pressure, intermediate pressure and freepower turbines 17, 18, 19 respectively drive the high and intermediatepressure compressors 15, 14 and the propellers 23, 24 by suitableinterconnecting shafts. The propellers 23, 24 normally provide themajority of the propulsive thrust. In the embodiments herein describedthe propellers 23, 24 rotate in opposite senses so that one rotatesclockwise and the other anti-clockwise around the engine's rotationalaxis 9.

With reference to FIGS. 2 and 3 the following reference directions areconsistently applied:

-   -   i) The axial direction 28 refers to a direction along the main        rotational axis of a gas turbine engine to which the fluidfoil        shown would be attached in use. The axial direction may        therefore be thought of as extending between a leading and        trailing edge of the fluidfoil in a line parallel to the main        rotational axis of the gas turbine engine.    -   ii) The radial direction 30 refers and to a direction        perpendicular to the main rotational axis of a gas turbine        engine to which the fluidfoil shown would be attached in use.        The radial direction 30 is substantially consistent with the        spanwise direction of the fluidfoil shown.    -   iii) Upstream and downstream refer to the principal flow        direction of fluid that would be normally present around the        fluidfoil shown when in use.

Referring specifically now to FIG. 2, a fluidfoil, in this case anopen-rotor propeller blade, is generally provided at 32. The blade 32 issuitable for use within the contra-rotating propeller stages 23, 24 ofengine 10. As will be appreciated the features discussed with respect toFIG. 2 (and FIG. 3 below) could equally be applied to fluidfoilssuitable for alternative purposes. Nonetheless fluidfoils such as blade32 may be particularly advantageous when used as the blades ofopen-rotor gas turbine engines, especially when used in a forwardpropeller stage 23 of a pair of contra-rotating propeller stages.

Blade 32 has a pressure surface 34 and a suction surface 36. The blade32 comprises a main body 38, a fence 40 and a blend region 42 disposedbetween the main body 38 and fence 40. The fence 40 is anhedral withrespect to the main body 38, that is the fence 40 is leant towards thepressure surface 34 of the main body 38. As will be appreciated, inother embodiments the fence may be dihedral or have both anhedral anddihedral portions. The blend region 42 provides a curved transitionbetween a tip 44 of the main body 38 and the fence 40, which is leantwith respect to the main body 38. Both the blend region 42 and fence 40retain the fluidfoil cross-section of the main body 38.

The blade 32 is divided into two portions, a first portion 46 nearest aleading edge 48 of the fence 40 and a second portion 50 immediatelyadjacent and downstream of the first portion 46 and extending to atrailing edge 52 of the fence 40. The first portion 46 and secondportion 50 are immediately adjacent. The two portions 46, 50 havedifferent lean angles. The lean angle of each portion 46, 50 correspondsto the approximate angle that the portion makes with the main body 38 ofthe blade 32.

More specifically the lean angle for the first portion 46 at theparticular chordwise position corresponding to the leading edge 48 is:90° plus an angle 54 subtended by:

-   -   i) a normal 56 to the pressure surface 34 at a point 58 on the        pressure surface 34 at the tip 44 of the main body 38 of the        fluidfoil 36 and at the particular chordwise position (in this        case the leading edge 48), and    -   ii) a straight line 60 between a tip 62 of the first portion 46        at the particular chordwise position (in this case the leading        edge 48) and the point 58 defined in i).

The lean angle for the second portion 50 at its upstream end (90° plusan angle 64) can be calculated in a similar way, taking account of theshift in the particular chordwise position.

Because the first portion 46 has a consistent lean angle throughout itsextent, the lean angle calculated for the leading edge 48 chordwiseposition will also be applicable at all other chordwise positionsthroughout the chordwise extent of the first portion 46. This is alsotrue for the second portion 50, albeit at a different consistent leanangle.

The lean angle of the first portion 46 is smaller than the lean angle ofthe second portion 50. The first portion 46 is therefore leant furthertowards pressure surface 34 than the second portion 50. Because the leanangle within each of the portions 46, 50 is consistent throughout thatportion 46, 50, there is a step-wise discontinuity 66 at the transitionbetween the two. In other embodiments however the portions 46, 50 couldbe provided with complimentary ramp regions which meet to smoothly jointhe portions 46, 50.

In the embodiment of FIG. 2, the blend region 42 has a radius ofcurvature of approximately 10% of the length of the chord of the mainbody 38 at its tip 44. The lean angle of the first portion 46 isapproximately 100° and the lean angle of the second portion 50 isapproximately 120°. In some embodiments the lean angle of the firstportion falls between approximately 80° and 120° and the lean angle ofthe second portion falls between approximately 100° and 140°. In theembodiment of FIG. 2, the spanwise length of each blade portion 46, 50is approximately 20% of the length of the chord of the main body 38 atits tip 44. In some embodiments the spanwise length of some or all ofthe blade portions fall between approximately 15% and 25% of the lengthof the chord of the main body 38 at its tip 44. In some embodiments athird portion (not shown) may be provided immediately downstream of thesecond portion. The third portion may also have a spanwise lengthbetween approximately 15% and 25% of the length of the chord of the mainbody 38 at its tip 44. Further the third portion may have a lean anglebetween approximately 80° and 120°.

When the blade 32 is rotating in use, air will flow around the blade 32in the axial direction 28, relatively high pressure existing adjacentthe pressure side 34 of the blade 32 and a relatively low pressureexisting adjacent the suction side 36 of the blade 32. The pressuredifferential would therefore tend to cause a flow of air from thepressure side 34 to the suction side 36 at the tip 44 of the main body38. As the air flows axially rearward from the leading edge 48, thefirst portion 46, with its relatively small lean angle, presents animpediment to fluid flow and decreases the flow tip momentum transferfrom the pressure side 34 to the suction side 36. The momentum transferdecrease, decreases the energy of a vortex formed by the blade 32towards the incipience of vortex breakdown. The second portion 50, withits larger lean angle, tends to reduce the impediment to fluid flow,allowing a modest increase in flow tip momentum transfer and preventingvortex breakdown from actually occurring. In embodiments where a thirdand possibly more portions are provided, these may be used to furthercontrol the momentum transfer with a view to maintaining the vortexenergy at the incipience of breakdown. This may be achieved by carefullyselecting the lean angle and/or spanwise length of the additionalportion(s).

The maintenance of the vortex at the incipience of breakdown may beparticularly advantageous where any vortex is likely to be interceptedby the blades of a downstream rotor (e.g. in the case of contra-rotatingrotor rows) or other structures. This is because a low energy vortex maygive rise to lower noise levels than a relatively high energy vortex orvortex breakdown when intercepted.

Referring now to FIG. 3, an alternative blade 70 embodiment is shown.The blade 70 is similar to the blade 32, but has a different fencearrangement. A fence 72 of the blade 70 has three portions; a firstportion 74, a second portion 76 and a third portion 78. The firstportion 74 is nearest a leading edge 80 of the blade 70, with the secondportion 76 immediately adjacent the first portion 74 and the thirdportion 78 immediately adjacent the second portion 76.

The lean angle of the fence 72 increases steadily from the leading edge80 to a trailing edge 82 of the blade 70. The lean angle variation iscontinuous and smooth throughout and across all of the portions 74, 76,78. Consequently there are no step-wise discontinuities between theportions 74, 76, 78, despite there being no provision of complimentaryramp regions. At the leading edge 80 the lean angle is approximately90°, while at the trailing edge 82 the lean angle is approximately 140°.

The second portion 76 is shorter than the first 74 and third 78portions. In the embodiment of FIG. 3, the spanwise length of the first74 and third 78 portions is approximately 25% of the length of the chordof a main body 84 of the blade 70 at its tip 86. In the embodiment ofFIG. 3, the spanwise length of the second portion 76 is approximately15% of the length of the chord of the main body 84 at its tip 86. Insome embodiments the spanwise width of each of the first 74, second 76and third 78 portions falls between approximately 15% and 25% of thelength of the chord of the main body 84 at its tip 86. In the embodimentof FIG. 3, blends 88 are provided to blend the different heights of thedifferent portions 74, 76, 78.

In the embodiment of FIG. 3 the shorter second portion 76 performs asimilar function to the second portion 50 of the FIG. 2 embodiment.Specifically, while the second portion 76 continues to present animpediment to the flow from a pressure side 90 to a suction side 92, theimpediment is reduced, allowing a modest increase in vortex energy andthe avoidance of vortex breakdown. The third portion 78, with restoredheight and a somewhat increased lean angle, tends to further refine thevortex energy, keeping it proximate incipient vortex breakdown. As willbe appreciated, the embodiments of both FIGS. 2 and 3 could be modifiedto include additional portions with a view to further refining controlof vortex energy. Further, for each portion, the spanwise length,chordwise width, lean angle and selection of anhedral or dihedral, aswell as other parameters, can be selected to suit a particularapplication or in order to provide particular vortex energy control inparticular operating envelopes.

It will be understood that the invention is not limited to theembodiments above-described and various modifications and improvementscan be made without departing from the various concepts describedherein. Except where mutually exclusive, any of the features may beemployed separately or in combination with any other features and theinvention extends to and includes all combinations and sub-combinationsof one or more features described herein in any form of fluidfoil or gasturbine engine.

1. A fluidfoil comprising a main body and a fence, wherein the fence hasdifferent lean angles with respect to the main body at differentchordwise positions.
 2. A fluidfoil according to claim 1 where the fenceis divided along its chordwise extent into at least two portions, wherea first portion nearer a leading edge of the fence has a smaller leanangle than a second portion further from the leading edge.
 3. Afluidfoil according to claim 2 where a third portion, further from theleading edge than the second portion, has a greater lean angle than thesecond portion.
 4. A fluidfoil according to claim 1 where a firstportion, nearest a leading edge of the fence, has the same lean angle asa second portion immediately adjacent the first portion and the secondportion is shorter than the first portion.
 5. A fluidfoil according toclaim 4 where a third portion, immediately adjacent the second portion,has a greater lean angle than the first and second portions.
 6. Afluidfoil according to claim 1 where the lean angle within at least oneportion is substantially consistent.
 7. A fluidfoil according to claim 1where the lean angle within at least one portion varies.
 8. A fluidfoilaccording to claim 1 where there is a step-wise discontinuity between atleast two of the portions.
 9. A fluidfoil according to claim 1 where atleast two portions have complimentary ramp regions which meet tosmoothly join the portions.
 10. A fluidfoil according to any claim 1where there is continuous smooth variation in the lean angle across thefence in the chordwise direction.
 11. A fluidfoil according to claim 1where the fluidfoil is an open-rotor propeller blade.
 12. A gas turbineengine comprising one or more fluidfoils in accordance with claim
 1. 13.A gas turbine engine according to claim 12 where the gas turbine engineis an open rotor engine.
 14. A method of managing the flow tip momentumtransfer from the pressure side to the suction side along the tip of afluidfoil comprising providing different degrees of impediment to fluidflow from the pressure side to the suction side at different pointsalong the tip in a chordwise direction.
 15. A method according to claim14 where the method further comprises arranging the impediments so thatin use they cause an approach to incipient vortex breakdown and maintainthis condition.